There exist many standards for encoding and decoding multimedia content. The content can include audio signals in one dimension, images with two dimensions in space, video sequences with a third dimension in time, text, or combinations thereof. Numerous standards exist for audio and text.
For images, the best known standard is JPEG, and for video sequences, the most widely used standards include MPEG-1, MPEG-2 and H.263. These standards are relatively low-level specifications that primarily deal with the spatial compression in the case of images, and spatial and temporal compression for video sequences. As a common feature, these standards perform compression on a frame basis. With these standards, one can achieve high compression ratios for a wide range of applications.
Newer video coding standards, such as MPEG-4, see “Information Technology—Generic coding of audio/visual objects,” ISO/IEC FDIS 14496-2 (MPEG4 Visual), November 1998, allow arbitrary-shaped objects to be encoded and decoded as separate video object planes (VOP). This emerging standard is intended to enable multimedia applications, such as interactive video, where natural and synthetic materials are integrated, and where access is universal. For example, one might want to “cut-and-paste” a moving figure or object from one video to another. In this type of scenario, it is assumed that the objects in the multimedia content have been identified through some type of segmentation algorithm, see for example, U.S. patent application Ser. No. 09/326,750 “Method for Ordering Image Spaces to Search for Object Surfaces” filed on Jun. 4, 1999 by Lin et al.
The most recent standardization effort taken on by the MPEG committee is that of MPEG-7, formally called “Multimedia Content Description Interface,” see “MPEG-7 Context, Objectives and Technical Roadmap,” ISO/IEC N2729, March 1999. Essentially, this standard plans to incorporate a set of descriptors and description schemes that can be used to describe various types of multimedia content. The descriptor and description schemes are associated with the content itself and allow for fast and efficient searching of material that is of interest to a particular user. It is important to note that this standard is not meant to replace previous coding standards. Rather, it builds on other standard representations, especially MPEG-4, because the multimedia content can be decomposed into different objects and each object can be assigned a unique set of descriptors. Also, the standard is independent of the format in which the content is stored. MPEG-7 descriptors can be attached to compressed or uncompressed data.
Descriptors for multimedia content can be used in a number of ways, see for example “MPEG-7 Applications,” ISO/IEC N2728, March 1999. Most interesting, for the purpose of the description below, are database search and retrieval applications. In a simple application environment, a user may specify some attributes of a particular object. At this low-level of representation, these attributes may include descriptors that describe the texture, motion and shape of the particular object. A method of representing and comparing shapes has been described in U.S. patent application Ser. No. 09/326,759 “Method for Ordering Image Spaces to Represent Object Shapes” filed on Jun. 4, 1999 by Lin et al. One of the drawbacks of this type of descriptor is that it is not straightforward to effectively combine this feature of the object with other low-level features. Another problem with such low-level descriptors, in general, is that a high-level interpretation of the object or multimedia content is difficult to obtain. Hence, there is a limitation in the level of representation.
To overcome the drawbacks mentioned above and obtain a higher-level of representation, one may consider more elaborate description schemes that combine several low-level descriptors. In fact, these description schemes may even contain other description schemes, see “MPEG-7 Description Schemes (V0.5),” ISO/IEC N2844, July 1999.
As shown in FIG. 1a, a generic description scheme (DS) has been proposed to represent multimedia content. This generic audio-visual DS 100 includes a separate syntactic DS 101, and a separate semantic DS 102. The semantic structure refers to the physical and logical signal aspects of the content, while the semantic structure refers to the conceptual meaning of the content. For a video sequence, the syntactic elements may be related to the color, shape and motion of a particular object. On the other hand, the semantic elements may refer to information that cannot be extracted from low-level descriptors, such as the time and place of an event or the name of a person in the multimedia content. In addition to the separate syntactic and semantic DSs, a syntactic-semantic relation graph DS 103 has been proposed to link the syntactic and semantic DSs.
The major problem with such a scheme is that the relations and attributes specified by the syntactic and semantic DS are independent, and it is the burden of the relation graph DS to create a coherent and meaningful interpretation of the multimedia content. Furthermore, the DSs mentioned above are either tree-based or graph-based. Tree-based representations provide an efficient means of searching and comparing, but are limited in their expressive ability; the independent syntactic and semantic DSs are tree-based. In contrast, graph-based representations provide a great deal of expressive ability, but are notoriously complex and prone to error for search and comparison.
For the task at hand, it is crucial that a representation scheme is not limited to how multimedia content is interpreted. The scheme should also provide an efficient means of comparison. From a human perspective, it is possible to interpret multimedia content in many ways; therefore, it is essential that any representation scheme allows multiple interpretations of the multimedia content. Although the independent syntactic and semantic DS, in conjunction with the relation graph DS, may allow multiple interpretations of multimedia content, it would not be efficient to perform comparisons.
As stated above, it is possible for a DS to contain other DSs. In the same way that the generic DS includes a syntactic DS, a semantic DS, and a syntactic/semantic relation graph DS. It has been proposed that the syntactic DS 101 includes a segment DS 105, a region DS 106, and a segment/region relation graph DS 107. As shown in FIG. 1b, the segment and region DSs may be used to define the temporal and spatial tree structure of multimedia content, respectively, and the segment/region relation graph DS may be used to describe the spatio-temporal relationships between segments and regions. Similarly, as shown in FIG. 1c, the semantic DS 102 includes an event DS 108, an object DS 109, and an event/object relation graph DS 110. The event and object DSs may be used to define event and object trees that define semantic index tables for temporal events and spatial objects, respectively. The event/object relation graph DS may be used to describe any type of spatio-temporal relationship between events and objects. As with the higher level DSs, namely the semantic and syntactic DSs, these lower-level DSs suffer the same problems with expressiveness and computational complexity.
Therefore, there is a need for representing syntactic and semantic attributes of multimedia content that balances the complexities of data structures and the methods that operate on the structures. In addition, there is a need to compare multimedia content according to content attributes.